1. Field of the Invention
The present invention relates to a digital signal coding method and apparatus, a signal recording medium, and a signal transmission method and in particular, to a digital signal coding method and apparatus, a signal recording medium, and a signal transmission method which can preferably be applied for a case of recording a moving picture signal and an acoustic signal on a recording medium such as a magneto-optical disc and a magnetic tape and reproducing the signals to be displayed on a display or for a video conference system or a broadcast apparatus for transmitting a moving picture signal and an acoustic signal from a transmission side to a reception side via a transmission path, so that the signals are received and displayed at the reception side.
2. Description of the Prior Art
A digital signal obtained by digitizing a moving picture signal, an acoustic signal, or the like has a plenty of information amount. Consequently, when recording the digital signal on a recording medium having a small storage information amount for a long period of time, or when transmitting the digital signal through a limited communication path with a plenty of channels, it is indispensable to provide means for coding the digital signal with a high efficiency. In order to answer such a requirement, there have been suggested high-efficiency coding methods using a correlation of a video signal. One of the methods is the MPEG method or MPEG specification. The MPEG (Moving Picture Image Coding Experts Group) was discussed and proposed as a standard plan in the ISO-IEC/HTC1/SC2/WG11. The MPEG specification is a hybrid method using a motion compensative predictive coding and a discrete cosine transform (DCT) in combination. In this MPEG method, firstly a video signal difference between frames is determined so as to remove a redundancy in the time axis direction and the discrete cosine transform is used to remove a redundancy in the spatial axis direction, thus enabling to encode a video signal with a high efficiency.
The MPEG specification consists of three parts: the MPEG video (ISO/IEC11172-2 or ISO/IEC13818-2) for a high-efficiency coding of a video signal; the MPEG audio (ISO/IEC11172-3 or ISO.IEC13818-3) for a high-efficiency coding of an audio signal; and the MPEG system (ISO/IEC11172-1 or ISO/IEC13818-1) for multiplexing compressed video and audio bit streams into a single bit stream and reproducing the stream with synchronization.
FIG. 1 is a block diagram showing a configuration example of a digital signal coding apparatus of a conventional MPEG method. Here, a feed forward control is carried out for determining an assignment (target) bit amount for each of input pictures when coding an input video signal. That is, a video signal S101 inputted from a terminal 200 is supplied to a frame memory 201. The frame memory 201 is capable of storing a predetermined length of an input video signal. Here, the predetermined time length is, for example, 0.5 seconds (15 frames in the case of NTSC). An image coding controller 202 reads a video signal S111 from the frame memory 201 and, for coding each of the pictures, calculates an assignment bit amount S112 and a picture coding parameter S117. More specifically, an assignment bit amount for a predetermined time length of a video signal is optimally distributed according to the images of the respective pictures. A video encoder 203 encodes a picture S102 supplied from the frame memory 201 according to the coding parameter S117, so that the picture S102 has the assignment bit amount S112 supplied from the image coding controller 202, and outputs a coded bit stream S103 of the picture S102 and a generated coding bit amount S113. The coded bit stream S103 is supplied to a buffer 204 and to an access unit detector 201. Moreover, the generated coding bit amount S113 outputted from the video encoder 203 is supplied to the image coding controller 202. In the image coding controller 202, if the generated bit amount is greater than the assignment bit amount, control is made so as to make the next assignment bit amount smaller; and if the generated bit amount is smaller than the assignment bit amount, control is made so as to make the next assignment bit amount greater.
An audio signal S105 inputted from a terminal 205 is supplied to an audio encoder 206. The audio encoder 206 encodes the input audio signal S105 on audio frame basis by way of the MPEG audio method and outputs a coded bit stream S106 to a buffer 207 and to the access unit detector 211. Here, one audio frame consists of 1152 samples in the case of the MPEG audio layer 2.
A sub picture signal S108 such as a superimpose information inputted from a terminal 208 is supplied to a sub picture encoder 209. The sub picture encoder 209 encodes each sub picture by way of the Run Length method, for example, and outputs a coded bit stream S109 obtained to a buffer 210 and to the access unit detector 211.
The access unit detector 211 detects a coding bit amount, a decoding time information, and a display time information of an access unit (decoding unit) of the video, audio, and sub picture coded bit streams and outputs the obtained information as an access unit information S114. Here, the access unit is one picture of a video signal, one frame of an audio signal, and one picture of a sub picture signal.
The video bit stream, the audio bit stream, and the sub picture bit stream are time-division multiplexed by the MPEG system method. The bit streams which have been multiplexed by the MPEG system method is formatted, for example, as shown in FIG. 2. In this FIG. 2, V. A, S respectively represent a video bit stream packet, an audio bit stream packet, and a sub picture bit stream packet. A packet has a configuration having a header information consisting of a decoding time information, display time information, and the like which is followed by a coded bit stream.
A multiplex scheduler 212, according to the access unit information S114, determines a schedule information S115 for time-division multiplexing the video, the audio, and the sub picture coded bit streams.
That is, the MPEG system method defines a system target decoder (STD) which is a virtual decoder model as shown in FIG. 3, and the multiplexed bit stream should be encoded while maintaining this system target decoder. This system target decoder (STD) shown in FIG. 3 will be detailed below.
In FIG. 3, a multiplexed bit stream is inputted from a terminal 301, and a switch 302, from this stream, isolates a video bit stream packet, an audio bit stream packet, and a sub picture bit stream packet, which packets are respectively supplied to a buffer 303, a buffer 305, and a buffer 307. A video decoder 304, an audio decoder 306, and a sub picture decoder 308 read out an access unit from respective buffers according to the decoding time information of the access unit of the coded bit streams stored in the respective buffers, and decode the access unit, so as to reproduce with synchronization the video , the audio, and the sub picture according to the display time information of the access unit. Here, the multiplexed bit stream should be coded so as to maintain the system target decoder, preventing overflow and underflow of the buffer 303, the buffer 305, and the buffer 307. The buffer overflow means that a bit stream storage amount exceeds a buffer capacity; and the buffer underflow means that an access unit is not entirely stored in the buffer by the decoding time of the access unit.
FIG. 4 shows a transition of a bit occupation in the STD buffer. In this FIG. 4, B represents a size of the STD buffer, and the vertical axis represents the buffer bit occupation amount or a data amount. A data input to the STD buffer is carried out with an input rate indicated by the inclination of the straight line ascending rightward, which increases the bit occupation amount. Moreover, in this figure, each of the access units A is instantaneously removed from the STD buffer at a corresponding decoding time DTS (decoding time stamp), which decreases the buffer bit occupation amount. The trace of the STD buffer bit occupation amount should be within the buffer size B.
The multiplex scheduler 212 of FIG. 1 determines a multiplex schedule of the video, audio, and sub picture coded bit streams, so as to prevent destruction of the system target decoder (STD), and outputs the information S115 to a multiplexer 213. That is, the multiplex scheduler 212 simulates the STD buffer bit occupation amount and determines the multiplex scheduler so as to prevent the buffer overflow and underflow. The mutliplexer 213, according to a mutliplex schedule information S115, reads out bit streams S104, S107, and S110 from the buffer 204, buffer 207, and buffer 210, respectively, and multiplexes the bit streams by way of time division, so as to output a multiplexed stream S116.
In the MPEG video method, it is possible to change a generated bit amount for each picture as well as a video coding bit rate and accordingly, the packet cycle of the video, audio, and sub picture in the multiplexed bit stream is not always constant. Generally, when determining a multiplex schedule of a current video packet, i.e., when establishing an input schedule of a current coding picture (access unit) to the system target decoder (STD), consideration is also taken on an information of a coding picture to be inputted to the system target decoder in future, which enables to obtain a save control.
That is, in FIG. 4, for example, in order to determine a video input schedule (input rate) to the STD buffer in a time window from the DTS[nxe2x88x921] to DTS[n], it is preferable to consider an information A[n] of coding of a current picture and an information of coding of several frames in future A[n+1], A[n+2] . . . DTS[n+1], DTS[n+2] . . . That is, in this case, by considering not only the multiplex schedule of the time window DTS[nxe2x88x921] to DTS[n] but also a multiplex schedule of the future, it is possible to secure a safe coding of a multiplexed stream.
For this, it is necessary to store a plurality of coded pictures (access units) in the buffer 204 and between the signals S103 and S104 there is often a delay amount corresponding to several frames required for determining the multiplex schedule.
FIG. 5 shows an example of a timing chart of a video signal processing in the digital signal coding apparatus of FIG. 1. Here, P1 to P10 represent pictures or their picture information. In this example, it is assumed that the feed forward control for determining an assignment coding bit amount for coding an input video is carried out to first five pictures in the future including a current picture. That is, in the frame memory 201, the input video signal S101 is delayed by a 5-picture time. Moreover, it is assumed that in order to determine a multiplex schedule of a current coding picture, a coding information on the first 5 pictures including a current picture is considered. That is, in the buffer 204, a delay of 5-picture time is required.
Here, as shown in FIG. 5, for the input video signal S101, the assignment coding bit amount S112 is delayed by 5-picture time. If it is assumed to ignore a delay amount in the encoder 203, the S11, the input picture S102 to the encoder 203, and the coded picture S103 are synchronized. Next, for the coded picture S103, the multiplex schedule information S115 is delayed by a 5-picture time. If it is assumed to ignore a delay in the multiplexer 213, the S115, the coded picture S104 which is read out from the buffer 204, and the multiplexed output S116 are synchronized.
Now, in the conventional digital signal coding apparatus of FIG. 1, the delay amount of the multiplexed bit stream S116 for the input video signal S101 mainly consists of a delay amount in the frame e memory 201 for calculating the assignment coding bit amount of a picture and a delay amount in the buffer 204 for the multiplex schedule calculation.
Generally, as the delay amount in the frame memory 201 fro calculation of the assignment coding bit amount increases, it is possible to drastically change the assignment coding bit amount of an input picture, enabling to obtain a coded picture of a high quality. However, this is accompanied by a greater delay amount in the buffer 203 for the multiplex schedule calculation, requiring a grater size (bit amount) of the buffer 204.
For this, conventionally, when it is desired to decrease the delay amount of the multiplexed bit stream S116 for the input video signal S101, the delay amount in the frame memory 201 or in the buffer 204 has been made small. However, if the delay amount in the frame memory 201 is made small, there arises a problem that the coded picture quality is deteriorated. Moreover, if the delay amount in the buffer 204 is made small, there arises a problem that it becomes difficult to establish a safe multiplex schedule.
It is therefore an object of the present invention to provide a digital coding method and apparatus, a signal recording medium, and a signal transmission method which enable to reduce a delay amount of an output multiplexed bit stream with respect to an input digital signal in comparison to a conventional digital signal coding apparatus as well as to obtain a coded picture of a high quality and to establish with safety a multiplex schedule for multiplexing coded bit streams.
In order to achieve the aforementioned object, the present invention is characterized in that when coding an input digital signal such as a video signal and an audio signal and multiplexing coded bit streams into a single bit stream, a coding control information for coding the input digital signal of a predetermined unit interval is adaptively determined and a multiplex schedule is controlled according to this control information, for multiplexing the coded bit stream.
Here, the coding control information includes an assignment coding bit amount and if the input digital signal is a video signal, the unit interval is a picture.
Because the multiplex schedule control is based not on an information such as a coding bit amount of an already coded signal but on the coding control information for coding, it is possible to reduce a memory capacity of a buffer which stores a coded signal.